The present invention relates to a roller train for enabling a slider to be freely guided along a track rail and to a linear motion guide apparatus employing this roller train that is used in machine tools, industrial robots, precision instruments, and the like.
Technology for enabling a sliding member to move reciprocatingly in a linear direction is indispensable for such fields as machine tools and industrial robotics. For example, a wafer grinder employed in semiconductor fabrication equipment is configured to move freely along a track rail disposed horizontally, performing coarse grinding at prescribed positions along the track rail and finishing grinding at other prescribed positions. For this reason, linear motion guide apparatuses have been widely used to enable the grinding device to move freely in a linear motion.
A variety of constructions for the linear motion guide apparatus described above are well known in the art. In general, however, this apparatus includes a long track rail; a slider capable of moving reciprocatingly along the track rail and comprising a main body and side covers fixed one to either side of the main body; and a plurality of rollers integrated in a path formed in both the slider and the track rail for supporting the slider on the track rail.
The main body of the slider is formed to fit on the track rail, loosely straddling the same. Rolling surfaces are formed on opposing surfaces of the main body and the track rail, respectively. These surfaces form a rolling channel along which rolling members can roll freely. The main body also includes through-holes formed through the length of the main body, return channels formed in the through-holes, and the inner portion of change direction channels connected one to each end of the return channels. A longitudinal section of the side covers on the slider has the same shape as a longitudinal section of the main body. The outer portion of the change direction channels is formed in the inner side surface of these side covers. Hence, the rolling channels, return channels, and change direction channels described above form endless paths having the shape of oval tracks. These paths are loaded with rollers, which are able to circulate freely in the channels.
Balls such as copper balls can be used in place of the rollers, however, the balls contact the track surface at points, whereas, the rollers contact the surface along lines enabling the rollers to support a greater load. Accordingly, the rollers are in high demand as they are generally used in devices that require supporting a relatively heavy load.
Next the operations of the linear motion guide apparatus having the construction described above and being well known in the art will be described. As the slider is moved in a sliding motion over the track rail, the rollers support the slider on the track rail and gradually circulate through the path. After each roller moves from the start of the rolling channel to the end of the rolling channel, the roller is transferred to the return channel via one of the change direction channels. The roller then proceeds through the return channel and is again transferred to the start of the rolling channel via the other change direction channel. The rollers positioned in the rolling channel support the slider, while the other rollers move through the change direction channels and the return channel along with the movement of the slider. For the purposes of description in the current application, the rolling channel in which rollers are supporting the slider will be referred to as the load-bearing region, while the other channels in which the rollers are simply moving and not supporting the slider will be referred to as the load-free region. Since the plurality of rollers continue to circulate throughout the path and support the slider on the track rail as described above, the slider can move smoothly and freely along the track rail.
However, in order for the linear motion guide apparatus described above to operate smoothly, smooth circulation of the rollers is most essential. Some factors that can prevent smooth circulation of the rollers include increased frictional resistance at points of contact between the rollers and the path or other components, as well as skew in the rotational axis of the rollers. In the present application, skew in the rotational axis of a roller is defined as the actual rotational axis of a roller in the path being at an angle to the designed rotational axis.
As shown in the example of FIG. 53, if the cylindrical roller 14a is skewed (the dotted line in the drawing shows the skewed position of the roller axis while the solid line shows the true axis), then when the cylindrical roller 14a is progressing from the end of the change direction channel 10 in the load-free region .beta. to the start of the rolling channel 5 of the load-bearing region .alpha., one half of the cylindrical roller 14a impacts the load-bearing region a before the other half of the cylindrical roller 14a, temporarily halting the circulation of the cylindrical roller 14a. Hence the skew in the cylindrical roller 14a hinders its smooth movement.
Furthermore, skew in the cylindrical roller 14a can generate an extreme concentrated load on one half of the cylindrical roller 14a, resulting in an edge load that can cause damage to the cylindrical roller 14a and the rolling channel 5, reducing their durability. Moreover, damage to the cylindrical roller 14a or rolling channel 5 can give rise to increased vibrations or rolling resistance on the cylindrical roller 14a as the same circulates through the path 12, further preventing the smooth motion of the cylindrical roller 14a.
Increased frictional resistance due to contact with other rollers or rough contact with part of the path can also hinder smooth circulation of the rollers. In general, such frictional resistance occurs mainly in the change direction channels.
Therefore, it is necessary to prevent skew in the rollers and increases in frictional resistance in order for the slider to move smoothly on the track rail. However, it is difficult to achieve smooth circulation of the rollers with the conventional construction described above, because no particular steps have been taken to prevent contact between the rollers and because the order of the rollers has a tendency to break down in the change direction channels, bringing rollers into contact with each other.
In the conventional construction described above, the side walls forming the return channel are employed as guiding surfaces to guide the rollers down the channel. However, in order for these side walls to serve as guiding surfaces, gaps of a fixed amount must be provided between each end surface of the roller and the corresponding side wall. However, these gaps give the rollers play in the channels, which may be a factor in generating skew in the rollers 14a. The problems described above are also applicable when employing spherical rollers or rollers with chamfered ends in place of the cylindrical rollers 14a.
In an effort to resolve the problems described above, Japanese Laid-Open Patent Publication No. HEI-10-110728 proposes an advanced technique aimed at preventing skew such as that shown in FIG. 54. This technique employs a retaining member 25 to link the rollers 14a in a train. As shown in the diagram, the retaining member 25 is formed in a ladder-like configuration and includes a plurality of roller retaining holes 26. Each roller retaining hole 26 retains one cylindrical roller 14a. A spacer 18 is disposed between each neighboring pair of rollers 14a for maintaining the positions of the rollers 14a. A protrusion 27 is formed on each outer side surface of the retaining member 25 and protrudes in the lengthwise direction indicated by the arrow X in FIG. 54(A) and in the direction from front to back in FIG. 54(B). Depressions (not shown) are formed in both side walls of the return channel and the change direction channels for engaging the protrusions 27.
Although the advanced technique described above succeeds in improving the effects for preventing skew in comparison to the conventional structure, the following new problems arise. When the rollers 14a pass through the semicircular change direction channels, the gap around the spacer 18 increases, while the rollers 14a are drawn toward the inner side of the change direction channel. As a result, the spaces between neighboring rollers 14a deviate, causing the line of rollers 14a in the change direction channel to deviate from that in the return channel. As a result the rollers 14a in the change direction channel have a tendency to press forcefully against a portion of the retaining member 25 (a portion of the roller retaining hole 26), thereby restricting circulation.
Moreover, the depression functioning as a guide surface must be formed with a high degree of parallelism and precise spacing throughout the entire return channel and change direction channels in order to achieve smooth movement of the rollers 14a using the advanced technique described above.
Furthermore, since the protrusions 27 are guided through the depressions in the change direction channels in a state of elastic deformation (a bowed state) corresponding to the curvature of the change direction channel, the protrusions 27 contact the depressions with considerable force. Therefore, frictional resistance is increased on the portions in sliding contact, thereby hindering the smooth circulation of the rollers 14a and preventing the smooth operation of the linear motion guide apparatus.